No one has heard before of Wu Bingyuan, a Chinese worker executed with a bullet in the back of his head during Mao Zedong's cultural revolution, but this review of the book which contains his picture should really be dedicated to him.

We know something about the top Communist party officials who fell foul of Mao, were targeted by the Red Guards and were driven to suicide or left to die of illness. We know next to nothing about the ordinary victims, men and women who defended their ideals against the Maoist dogma or simply refused to conform to the hysteria.

Wu was one of those, a technician at the Harbin Electric Motor Factory who, with a colleague, wrote a pamphlet called Looking North. It was interpreted by the authorities as criticism of Mao's hostile policy towards the Soviet Union - China's northern neighbour - and the punishment for this "counter-revolutionary" act was death. On April 5 1968, the photographer Li Zhensheng took a set of official pictures of Wu's execution, along with that of his fellow technician and six "ordinary" criminals. The eight were sentenced to death at a public trial, placed on trucks with placards round their necks, driven to a cemetery, forced to kneel and shot before a large crowd.

Similar events happened all over China and (minus, in most cases, the public execution) still happen today. But Wu made a small heroic gesture that Li's pictures have rescued from anonymity. When sentence was first pronounced, he looked at the sky and uttered one sentence: "This world is too dark." Then he closed his eyes and kept them defiantly shut till he had left the dark world.

This collection of photos, taken by Li in the north-eastern province of Heilongjiang, where he worked for the official Communist party newspaper, is unique for a simple reason. Although the post-Mao Chinese government has labelled the cultural revolution "10 years of chaos", it still tries to suppress any real inquiry into the countless human tragedies it caused. After all, the Communist party continued to rule China during those 10 years, and some of those implicated then are still in power (for example in Tibet, where they are a serious obstacle to any settlement with the Dalai Lama). Even today, the Chinese media are not allowed to recall the tragic stories of those years - such as the suicide of an opera singer in the Shanghai lane where I lived until recently. Only my neighbours remembered how the Red Guards humiliated her on her doorstep; she was found hanging in the bathroom the next day.

Li was a Red Guard as well as a photographer, and does not deny that he also led "struggle sessions" against innocent victims; but his pictures reflect a deeper desire to record and understand. When he in turn was "struggled against", he concealed the negatives under his floorboards. He published a very few in 1988 during the more liberal period before the Tiananmen Square clampdown. Other pictures have appeared since then, but only abroad: the present album, compiled after several years of editing with Li's collaboration in New York, contains 300 photos out of the 30,000 he hid.

The story begins in 1965, a year before the cultural revolution, when the rural people's communes were already convulsed by a campaign to root out former landlords and other "bad elements". One "rich peasant" wearing a tattered jacket bows his head before a huge crowd squatting in the main square of a small rural town: his accusers sit grinning in the foreground. A hundred or so other "bad elements" wait their turn elsewhere under armed guard, in a slightly out-of-focus picture that accentuates their air of hopelessness.

A year later, in the provincial capital of Harbin, Li films a far more violent struggle session by Red Guards against disgraced party leaders. They stand, wobbling awkwardly, on fold-up chairs, with placards round their necks. One of them, former governor Li Fanwu, had the imprudence to grow his hair long like Chairman Mao. A young girl, her chubby face rigid with revolutionary zeal, shears off his locks and stuffs them down his neck. Li's daughter was offered a good job in the army if she would sign a document accusing him of committing incest with her. She accepted.

Another year and the Red Guards are fighting among themselves for control of a loudspeaker bus. Li photographs a young man on the point of death, eyes shut and face bloodied. The Red Guards are sorted out by the army and sent off to the countryside: their random violence is institutionalised by a new clique of "revolutionary" leaders. Li's pictures show atrocious acts carried out not in a frenzy but with stony deliberation. One victim has his mouth stuffed with a glove to prevent him protesting: both he and his tormenters appear to be posing for the camera. (Later he officially commits suicide - or is pushed out of a window.)

Few people smile in these pictures unless they are hailing the Red, Red Sun in our Hearts (Chairman Mao). There is one important exception. Shortly before the mass execution at which Wu Bingyuan was shot, Li gets married. His colleagues playfully drape the newlyweds' necks with placards like those hung over the "criminals". Instead of saying "counter-revolutionary element sentenced to immediate death", the placards proclaim that the bride and groom are "taking the socialist road".

There was more to the cultural revolution than the violence (which was most severe during the years 1966-69). We may discuss the often genuine idealism of the young people who volunteered to "serve the people". "Barefoot doctors" took basic medicine to the villages; high school graduates took education to remote communities. We may reflect on the elements of an alternative road to socialism that sought to reject bureaucracy and privilege, and impressed many who sought similar alternatives abroad. Yet these "spring shoots" were killed off by the stifling cult of Mao and the tyranny of the ambitious and the ignorant to which he turned a blind eye. Their violence was corrosive and endemic - not the occasional "excess" for which it was too easily excused.

This remarkable book, which still cannot be published on the mainland, is a salutary reminder that, in the Chinese phrase, accounts have yet to be settled with the past: Li's photographic record ends with a final chilling scene, taken four years after the cultural revolution ended. Wang Shouxin, one of the most active "rebels" when it was raging, is now in turn sentenced to death. Wang does not sound a lovely character, but was it really necessary to dislocate her jaw so she could not proclaim her innocence before being shot in the head? There is an ambiguity here, as Professor Jonathan Spence notes in his foreword, which means that we - and the Chinese people - still need to ask questions about their hidden past.

John Gittings recently returned from Shanghai after 25 years reporting for the Guardian on China.